Data on nitrogen in groundwater and surface waters in the sewered and unsewered parts of Nassau County were analyzed to evaluate the principal sources of nitrogen and to determine what factors significantly affect its concentration in the upper glacial aquifer and the underlying Magothy aquifer. Nitrate data for water samples collected during 1952-76 show that: (1) median nitrate concentration of water in the sewered part of the entire thickness of the upper glacial aquifer is not significantly different (at the 0.95 confidence limit, N = 1400) from that in the unsewered area, (2) median nitrate concentrations in the upper 3 meters of the upper glacial aquifer are significantly lower (at the 0.90 confidence limit, N = 15) in the sewered area than in the unsewered area, (3) in the sewered area, water from 8 of 10 wells in the upper glacial aquifer shows significantly decreasing nitrate concentration with time, (4) total nitrogen concentrations in streams draining the sewered area are significantly lower than in those draining the unsewered area, (5) from more than 2,000 nitrate analyses of water in the Magothy aquifer collected from 1952-76, it was found that nitrate is present at all depths but that nitrate concentrations decrease with depth, and (6) although water from most Magothy wells having long-term nitrate records shows no significant change in nitrate concentration with time, where the change is significant, nitrate is increasing. (USGS)
